Open top defluxers are commonly used for removing oil and grease from metal and other parts Such an open top defluxer generally comprises an open topped tank formed from plural upright sidewalls connected to a bottom wall and into which a liquid solvent is disposed. Cleaning vapors are generated by boiling the solvent with heating elements immersed therein. The generated vapors rise within the tank to contact the parts to be cleaned which are supported in a solvent vapor zone within the tank above the liquid solvent. The vapors will dissolve the oil or grease on the parts and, optionally, the parts may be further cleaned by immersion in a rinse sump in the tank containing relatively clean liquid solvent.
Because the light rising solvent vapors have a natural tendency to escape from the tank by following a natural convection flow path upwardly along the tank sidewalls through the open top, it is desirable to control the flow path of the vapors before they can be expelled into the surrounding ambient environment. A very well known solution is the provision of a cold condenser in the form of a cold water jacket encircling the inner surfaces of the tank sidewalls which define an upper limit or a vapor line of the solvent vapor zone. The condensing coils advantageously prevent most of the light solvent vapors from rising and escaping through the open top by providing a chill zone for condensing these vapors. The condensing coils also prevent the metal walls of the defluxer from warming since, if the walls are allowed to warm, light solvent vapors are encouraged to rise and escape from the open top.
To further prevent the light solvent vapors from escaping, the tank sidewalls extend upwardly for a certain distance above the vapor line to define a freeboard region which is the height or distance between the vapor line and uppermost edges of the tank sidewalls. This freeboard region provides an additional area in which light solvent vapors rising from the vapor zone can condense without escaping from the open top.
Notwithstanding the above-identified known solutions for limiting the outflux of solvent vapor from the open top, there is still a tendency for the lighter vapor generated from the solvent to pass both the cooling coils and the overhead freeboard region and escape to the atmosphere by primarily flowing along the inner surfaces of the upright sidewalls and out through the open top. Such escape of solvent vapors results in excessive loss of solvent which renders the defluxing operating costly and results in noxious solvent odors
from the defluxer causing a toxic hazard to personnel through air pollution.
At present, CFC-113 type solvents are commonly used in cleaning and defluxing of, for example, printed circuit boards following a soldering operation of component parts to the board. Since these CFC-113 solvents are substantial contributors to depletion of the ozone layer, new HCFC solvents having less deleterious harmful environmental effects than the CFC-113 solvents are becoming popular. However, these new HCFC solvents have lower boiling temperatures than the CFC-113 solvents and are more conducive to excessive loss by evaporation and escape to the atmosphere. Accordingly, there exists a greater need for controlling the loss of generated solvent vapors by the use of the new HCFC solvents in comparison with the CFC-113 solvents.
To limit the outflux of the solvent vapor from an open top defluxer, the present inventors initially experimented with the concept of extending the height of the freeboard region of the open top defluxer so that the freeboard region extended considerably above the typical engineering design criteria of 75% of the tank width. By extensive experimentation, however, and unexpectedly, the present inventors discovered that there exists an optimal freeboard region height, which is different for both the CFC-113 as well as the new HCFC solvents, above which optimal height the extent of solvent loss actually increases, presumably as a result of an increased "chimney effect." Therefore, as a result of such extensive experimentation, the inventors learned that a need exists for optimizing the height of the freeboard region in both existing and newly manufactured open top defluxers so as to enable both types of defluxers to work efficiently with either the existing CFC-113 solvents or the new HCFC solvents, as well as with other solvents having different boiling temperatures, while minimizing solvent loss.